The present invention refers generally to the field of electronics and more specifically to output buffers for electronic circuits.
Output buffers are generally exploited for interfacing electronic circuits with the other external or outside circuits. For example, in an electronic system such as a computer system, which includes different circuits, the circuits interact with each other by means of a so-called system bus. Each electronic circuit of the system typically includes one or more output buffers, necessary for electrically insulating the circuit from the system bus. The output buffer performs additional functions, like amplifying the signals that have to be made available to the system bus and providing the electrical power (i.e., the current) needed for driving the capacitive load that is usually introduced by the bus. A very important parameter of each output buffer is its driving capability. With the term driving capability it is intended the highest speed with which a signal generated by the output buffer may switch.
For the purpose of assuring optimal performance, the driving capability of the output buffer has to take a specific value (possibly, adjustable according to the capacitive load).
However, the driving capability of an output buffer depends on the process parameters used for its production. Since such process parameters, like the gain and the threshold voltage of the transistors, may be subjected to non-negligible spreads, the driving capability of each output buffer may have a different and not predictable value. Furthermore, the driving capability depends on operative parameters, like the supply voltage and the temperature. The operative parameters may vary over time during the operation of the output buffer and, as a consequence, the driving capability may similarly vary over time.
A known solution provides the dimensioning of the output buffer according to the worst case operating conditions (i.e., with the lowest supply voltage, the highest temperature, the highest threshold voltage of the transistors and the lowest gain of the transistors). This guarantees that the output buffer has a driving capability always higher than the desired value.
However, the possibility of being in the worst operating condition for all the parameters is remote. Consequently, in the majority of the cases, the driving capability will be too high, thereby degrading the signal to noise-ratio of the output buffer. For this purpose, two solutions are usually exploited.
According to a first known solution, a continual monitoring of the supply voltage is performed, in such a way to correct the driving capability according to its variation. However, the correction that can be performed may be not accurate, since it is not known a priori the amount of current that each transistor can generate depending on its supply voltage. A second solution known in the art provides the realization of an output buffer structured as a current mirror. Indeed, ideally, the current mirror circuit structure is immune to the variations of temperature, process and supply voltage. However, such circuit structure is characterized by a response speed that is too slow for the specifics usually required for a modern output buffer.